The previous association of two genes (i.e. wcrF and allelic wcrC) with different linkages between Gal and ribitol-5-phosphate in Streptococcus oralis receptor polysaccharides (RPS) and S. pneumoniae capsular polysaccharides (CPS) provided the necessary molecular basis for predicting the structures of CPS10B and CPS10C from the known structures of CPS10A and CPS10F, respectively. We have now isolated and structurally characterized CPS10B and CPS10C from S. pneumoniae strains. As predicted, the presence of a 1-2 linkage between Gal and ribitol-5-phosphate of CPS10C distinguished this serotype from CPS10F. Likewise, the presence of a 1-4 linkage between these units of CPS10B distinguished this serotype from closely related CPS10A. However, in addition to this difference, CPS10B was found to lack a Galp branch like the one present in CPS10A. This was unexpected since the gene (wcrG) for this branch was present in the cps10B locus. Insertional inactivation of wcrG in the cps10A locus eliminated the 1-6 linked Galp branch from CPS10A;however, similar inactivation of this gene in the cps10B locus failed to alter the structure of CPS10B, thereby indicating that in this serotype, wcrG is silent. In addition to wcrG, we disrupted wcrD in both the cps10A and cps10B loci and observed the production of immunoreactive polysaccharides by the resulting mutant strains. Structural characterization of these polysaccharides is in progress to complete the molecular characterization of CPS serogroup 10. We anticipate that results from this work will provide new insights into the evolution of this CPS serogroup of pathogenic S. pneumonia to RPS of oral commensals such as S. oralis. Colonization of the tooth surface by gram-positive Actinomyces oris has long been known to involve two antigenically and functionally distinct types of fimbriae. Type 1 fimbriae mediate attachment of actinomyces to adsorbed salivary proline-rich proteins (PRP) whereas type 2 fimbriae bind specific GalNAc- and Gal-containing saccharide motifs present in both streptococcal RPS and in host glycoconjugates. Although the distinct binding specificities of these fimbriae are well established, identification of the corresponding adhesins has not been possible due to the uncertainties in the subunit structures of these organelles. However, we now know that each type of fimbria is synthesized from an operon that contains three genes: fimQ-fimP-srtC1 for type 1 fimbriae and fimB-fimA-srtC2 for type 2 fimbriae. We also know that cell surface production of these structures depends on specific transpeptidases referred to as sortases (i.e. SrtC1 or SrtC2), which catalyze the covalent polymerization of tip and shaft fimbrillins designated respectively, FimQ and FimP for type 1 and FimB and FimA for type 2 fimbriae. In recent studies, mutants lacking srtC2, fimA or fimB were isolated following development of a facile genetic system for creating in-frame deletion mutants of A. oris. The srtC2 and fimA deletion mutants, both of which lacked high molecular weight type 2 fimbriae, were non-adherent as indicated by their failure to cause coaggregation of S. oralis 34 or hemagglutination of sialidase-treated RBC. Adhesion of these mutants was restored by plasmid-based expression of each deleted gene (i.e. srtC2 or fimA). In striking contrast with the phenotypes of these mutants, the fimB mutant coaggregated strongly with S. oralis 34 and mediated hemagglutination of sialidase-treated RBC. Further characterization of this mutant revealed cell surface fimbriae composed solely of FimA. Thus, the shaft fimbrillin FimA rather than the tip fimbrillin FimB accounts for the lectin-like activity of these fimbriae. Microbial colonization of oral surfaces depends on bacterial binding of surface-associated receptors in the presence of soluble receptor molecules that are potential competitive inhibitors of adhesion. The underlying basis of this selectivity has been explained in different ways. Thus, type 1-fimbriae-mediated adhesion of actinomyces to surface-associated PRP in saliva, which is rich in soluble PRP, was suggested to depend on the specificity of the type 1 fimbrial adhesin for cryptic features of PRP molecules that are exposed only when these proteins adsorb to the tooth surface. In contrast, a greater avidity of fimbriated actinomyces for surface-associated receptors (e.g. cell-surface RPS) than for soluble receptor molecules such as salivary mucins was suggested as the possible explanation for type 2 fimbriae-mediated adhesion. Recently, a third possible explanation for adhesion in the oral environment emerged from studies of how variations in shear force affect fimbriae-mediated adhesion of Escherichia coli. In these studies, a reversible change in adhesion strength was postulated to arise from a shear-dependent conformational change in the fimbrial adhesin (FimH), associated with formation of a so-called catch-bond that increased the strength of the adhesin-ligand interaction. This increase provided a novel explanation for stronger binding of surface-associated than soluble receptors as the latter move with the bulk flow and thus do not induce catch-bond formation. We have now examined the affect of shear on type 1 and type 2 fimbriae-mediated adhesion of Actinomyces spp., as well as on Hsa-mediated adhesion of S. gordonii to sialic acid-containing receptors. Evidence of shear-enhanced adhesion, although not obtained in studies with actinomyces was obtained for Hsa-mediated adhesion of S. gordonii. Thus, streptococci were seen rolling across these receptor-coated surfaces at low flow rates but became firmly attached at higher flow rates. The reversible nature of this transition was demonstrated by increasing the shear on attached streptococci and then returning to the starting value. In response, a sizeable fraction of the rolling cells became stationary at the higher shear and resumed rolling when shear was reduced. These findings suggest that Hsa of S. gordonii is a catch-bond adhesin. In addition to promoting adhesion in the oral cavity, Hsa has been implicated in the pathogenesis of infective endocarditis. The present findings clearly raise the possibility that shear-enhanced adhesion contributes to the persistence of streptococci in turbulent environments such as in the endocardium.